This invention relates to a method of preparing a support for a printing plate from an aluminum plate of which the surface is electrochemically roughed.
In general, aluminum plates are used as the support for offset printing plates of which the surface is usually roughed in order to improve adhesion to the photosensitive layer provided thereon, to hold damping water used during printing, and the like.
As the roughing means, mechanical treatments, such as ball graining and brush graining, are known. Recently, electrolytic roughing has been developed wherein the surface of the aluminum plate is roughed electrochemically in an acidic electrolyte solution, such as hydrochloric acid or nitric acid. The surface of the aluminum plate treated with the electric roughing is uniformly roughed with a narrow means roughness distribution compared with the conventional mechanical roughing but the conditions capable of forming the above roughed surface are very restricted. That is, aluminum plates, of which the quality and properties are uniform, can readily be obtained by maintaining constant various conditions, such as the composition of the electrolyte solution, temperature and electrolysis conditions.
However, in the electrolytic roughing conducted in the aqueous solution of hydrochloric acid, nitric acid or the like, the solution is depleted by the aluminum ions eluted by the electrochemical reaction. According, it is necessary to control the aluminum ion concentration and to maintain the hydrochloric acid or nitric acid concentration of the electrolyte solution constant by adding water and hydrochloric acid or nitric acid. Furthermore, it is necessary to eliminate the components, which are harmful to the natural environment, from the electrolyte solution prior to discharging it as a waste liquid. Because of the above reasons, the conventional electrolytic roughing, which is conducted in the aqueous hydrochloric acid solution or the like, was very expensive. The acid can be recovered by using an ion-exchange membrane, but this method requires extensive auxiliary equipment and has a problem in maintenance.
In order to improve the above problems of the electrolytic roughing conducted in the aqueous hydrochloric acid solution or the like, a method proposed is of conducting the electrolytic roughing in an aqueous neutral salt solution by using direct current or alternating current (GB 1532303). This method is preferable in view of low cost, but is insufficient for producing uniform sufficiently deep pits.